How to render a very complex scene using Render Layers?
Okay, I'm going to get the ball rolling by answering your question at face value. Here's a simple alpha over composite with 2 Render Layers.

I have 2 Scene Layers active, and have assigned each scene layer to a render layer. You'll notice that the shadow is rendered on the ground layer. This is because both layers go into memory and the monkey casts a shadow on the ground even though the monkey is not rendered in  layer one. If (for memory reasons) you did not want the monkey to affect the render calculations for Render Layer 1, you would add an Exclude selection for layer 2 on the Ground Render Layer. If you did this it might help with the rendering load, but you would lose your shadow on the ground, and would have to render it another way. If your ground object was really complicated and heavy (rendering wise), you could add a second simple plane on a third layer that could be used to generate the shadows for your solider layer, and the composite the 3 layers together.